As the quest for clean, abundant and secure energy supply intensifies, attention is being increasingly directed to marine sources such as wind, waves and ocean currents. Each of these resources is a concentrated kinetic form of solar energy capable of providing a significant share of global human demand. A multitude of technologies has emerged to harness and convert these powerful forces. Every technical proposal is subject to the same realities, namely that to be economically viable, its physical structure must capture the maximum possible energy with the lowest possible materials cost and over the greatest possible lifespan. Additionally, the frequency and expense of maintenance must be minimized. In the harsh marine environment, each of these factors presents a particularly acute challenge.
The most mature technology presently deployed offshore is that of wind turbines. Locating offshore is advantageous for two main reasons. First, the wind speed over water is less impeded by surface friction and obstacle-induced turbulence than it is on land, allowing substantially higher energy potential. Second, land-based wind farms are limited by available land area, and elicit considerable social resistance due to perceived “visual blighting” of the landscape. Unfortunately, these offshore wind installations have encountered substantial obstacles. Though visual pollution is less of an issue, because of the required minimum inter-turbine spacing, very large areas of towers still fill the horizon. There are physical issues, as well. Due to the economically-driven trend towards extremely large blade rotor diameters, and the resultant concentrated stresses at the blade roots, hub and gear mechanisms, component failures requiring expensive overhaul are common. This high rate of wear also directly affects turbine lifespan which is currently projected to be approximately 20 years. A further consequence of these stress loads is that conventional wind turbines are approaching the upper limits of unit size, which increases the footprint area of the wind farm for any given output.
There exist two major types of wind turbine: Those whose blades radiate from a hub on a horizontal shaft, (Horizontal Axis Wind Turbines, or HAWTs), and those with a vertical axis of blade rotation, (Vertical Axis Wind Turbine, or VAWTs). The present invention relates to an interpretation of the VAWT type of machine. At present, there are several known structures with a number of vertical vanes supported by a system of horizontal spokes radiating from a hub which is supported on top of a bearing structure, and which hub, in turn, transfers a torque from the vanes to a vertical shaft. The drawbacks to this configuration are twofold. First, the horizontal spokes serve no constructive aerodynamic purpose; in fact, they contribute only parasitic drag, which limits turbine efficiency. Additionally, as the diameter of the turbine increases, the mass of said horizontal spokes increases in a non-linear fashion, further limiting turbine efficiency, and ultimately structurally limiting the overall diameter and output of the turbine. Second, supporting the turbine assembly on a central hub concentrates load forces over a small bearing surface, which causes significant wear, and consequently reduces system lifespan.
Various VAWT designs have been proposed that eliminate the horizontal support spokes and central hub, instead mounting the wind-propelled vanes on some form of peripheral bearing structure. The greatest significance of the hubless VAWT design is that the area presented to the wind, and the subsequent power output is no longer restricted by the physical limits of blade length as it is with HAWTs, or by the aerodynamic drag and mass of the horizontal spoke arrangement of conventional VAWTs. Consequently, the theoretical scale of the hubless VAWT is virtually unlimited. It should be noted, however, that in practical terms the cost of an offshore floating hubless VAWT may be prohibitively great. In this context, the advantage of symbiotically integrating the architecture of wave and ocean current energy systems with this class of VAWT, thereby sharing costs, is compelling.
Another unavoidable aspect of offshore wind farms common to all marine-based systems is the necessity for power transmission back to land. High voltage submarine electricity cables or Hydrogen gas pipelines represent a significant capital cost which directly affects profitability. Because the output of an individual conventional wind turbine is relatively small, the energy transmission network linking multiple units is complex and extensive, resulting in substantial costs and environmental impact. Conversely, the larger the output of an individual turbine, or hybrid, the simpler the transmission network becomes, reducing costs and environmental disruption.
A controversial, but nonetheless valid environmental concern is that of bird kill. At issue is the fact that although they are rotating at only 13.4 rpm, and although the blade roots are clearly visible, the slender blade tips of a 100 m diameter HAWT rotor travel at 252 Km/h, inhibiting blade visibility at the outer region of the swept area, and increasing the likelihood of collision. By contrast, both the blade roots and tips of a large-diameter VAWT move at the same low rotational speed, enhancing overall visibility and decreasing the likelihood of collision.
The major drawback to wind farms is their unpredictable intermittency. Unlike wave conditions, which can be forecast days in advance, and ocean current flows which are inherently stable and reliable, wind conditions can fluctuate radically within hours. The practical consequence of this is that backup fossil fuel power plants with capacity equivalent to that of the wind farm, must be maintained in anticipation of a sudden drop in wind energy output. By forming a hybrid of diverse marine energy sources, there is less fluctuation in output, requiring less supplemental fossil-derived backup.
The second major contender for marine energy production is wave power. Despite a multitude of proposed designs over the past 200 years, practical development has been stymied by the emergence and exploitation of cheap, relatively convenient fossil fuels. As with all renewable energy sources, wave power is for the first time enjoying the research and investment required for commercialization. Because wave energy is more densely concentrated than wind energy, the potential exists to generate the equivalent output of a wind farm whilst occupying less than a quarter of the surface area.
There are two major types of wave energy converters (WECs), “point source”, and “attenuators”. Point source WECs extract energy from a relatively small unit of surface area per device, whereas attenuators generally span several wave crests concurrently, and occupy a greater area per device. The present invention may be configured to integrate either the point source, or attenuator-type of WEC, or a combination thereof, into the primary structure, but may also serve as a mooring facility for a diversity of different type WECs. In fact, a circumferentially-located attenuator-type WEC, particularly if composed of a soft, pneumatic structure could serve as a protective barrier surrounding the present invention. The most common point source WEC is known as an “oscillating wafer column”. In this design, the rise and fall in water pressure of waves is used to alternately push and pull a column of air within some form of duct, ultimately driving a pneumatic turbine.
Despite the promise of wave energy converters, the vast majority of existing designs share a common set of drawbacks that negatively affect capital and operational costs. First, almost without exception, WECs are very material intensive. The major reason for this is the necessity for “reactive mass”, namely a physical mass equal to the wave mass which is reacting against it. A secondary reason for heavy construction is survivability in high seas. The cost of WEC structural mass can account for as much as 80% of capital cost. Another disadvantage of present designs is they tend to consist of fields of multiple units, similar to wind farms. Consequently, large areas are occupied, mooring and power transmission networks are extensive, shipping navigation is impeded, and maintenance of individual units at sea involves high risk and expense. Another tendency of WEC designs is that towards complexity of mechanical-to-electrical conversion. Many proposals involve stress-prone linkages and hydraulic circuits. As a result, system lifespan is usually projected to be less than 20 years.
The third marine energy source under consideration is ocean current, both tidal and deep sea. The overwhelming advantage of marine current flow is its reliability and predictability. Unlike wind or wave regimes, steady-state ocean currents such as the United States' Gulf Stream and Japan's Kuroshio can produce clean power as reliably as fossil fueled plants. This concept is defined as capacity factor (CF), which is a function of actual system output over time relative to 100% of theoretical potential output over the same period. Nuclear power plants are capable of a 90% CF, and coal-fired plants typically run at 70% CF. By comparison, offshore wind farms and wave farms each appear capable of a CF of 45%. In this context, the significance of an ocean current power plant achieving a CF of 90%+ is apparent. Another benefit of water turbines is that because at sea level, water is 784 times denser than air, a water-driven turbine is substantially smaller for an equivalent power output.
The fundamental objective that ocean current turbines share with wind turbines, is that a maximum area of flow should be intercepted utilizing the minimum material investment. The identical rationale for the hubless vertical axis wind turbine (VAWT) above water can be applied to its submarine ocean current counterpart.
Ocean current converters share some of the limitations common to wind and wave devices. These include a reliance on large areas of deployment necessitating extensive mooring and transmission networks, and high risk, expensive maintenance of individual units. In addition, ocean current poses some unique design challenges. Among these is the biofouling of slender hydrofoils which reduces efficiency and increases maintenance costs. Another concern is the threat of these hydrofoils to large marine mammals. Finally, system life spans are not likely to exceed the present twenty year projections for either wind or wave based schemes.